Current wind turbine blade certification requires performing several static and dynamic tests on the blade to confirm the blade capability to withstand the expected loads, particularly static strength tests to check the blade behaviour under extreme design loads, fatigue tests to check the blade's ability to withstand the operating loads for its design life and other tests in order to characterize physical properties of the blade such as weight, centre of gravity or natural modes and frequencies.
These tests are usually carried out in a test bench in which the blade is bolted to a rigid structure and then static or dynamic loads are applied to the blade by means of different methods.
Most of the currently used test systems rely on a set of strain gauges to determine the strain distribution along the blades. With the increasing size of blades, the amount of necessary gauges is rapidly growing, which implies larger efforts to adequately estimate the aforementioned distribution. With two to four electrical wires per gauge, cabling volume, complexity and vulnerability to electro-magnetic interference become significant and disturbing. Additionally, the discrete nature of this kind of measurements implies loss of information in case of non-linear behaviour in the area between two strain gauges.
WO 2010/086466 in the name of the applicant discloses a strain measuring system of wind turbine blades during the performance of static tests that comprises an equipment for measuring the strain at multiple locations in mono-mode optical fibres using Rayleigh scattered light, that includes an Optical Backscattering Reflectometer (OBR) interrogator, an interface device and an Acquisition System, said mono-mode optical fibres being attached to the blade subjected to said tests for obtaining high spatial resolution measurements of the blade strain during said tests.
There are physical properties of the blade such as the longitudinal mass distribution or the bending stiffness distribution in different zones of the blade that cannot be well characterized using the above mentioned technology based on strain gauges due to its low spatial resolution. On the other side there are not known testing methods with such purpose using the technology disclosed in WO 2010/086466.
However a good knowledge of said physical properties at the manufacturing plant would be very useful in order to establish statistic quality controls over the entire population for detecting local changes of stiffness and/or mass, generated by defects or errors during the production process. It will be also very useful in many other stages of the blades lifetime.
There is therefore a need of new testing methods for determining the physical properties of the blades and this invention is intended to its attention.